Heaven's Clothesline

Tethers Unlimited's illustration shows how a relay of space cables might boost a payload from a hypersonic airplane.Illustration courtesy of Tethers Unlimited.

For decades, engineers have touted the potential of the space tethera simple-sounding technology that consists of little more than a cable attached to an orbiting spacecraft. Experiments during the 1990s demonstrated that tethers could be used to discard old satellites into the atmosphere or to maneuver satellites into higher orbits by pushing or pulling against Earth's magnetic field. Now NASA is doling out funds to transform the space tether from a curiosity to a practical tool.

Next February a group headed by NASA and the Harvard-Smithsonian Center for Astrophysics plans to launch ProSEDS, a 3.1-mile-long wire linked to a 6.2-mile tether, which in turn is attached to the second stage of a Delta II rocket. The wire end of the tether will dangle into Earth's ionosphere, sucking up electrons. The resulting current creates electromagnetic drag, which pulls the whole assembly into a lower orbit. NASA may adapt the technique in reverse, running current through a tether to boost the orbit of the International Space Station.

Other teams are studying even more ambitious types of tethers. Robert Hoyt, a physicist at Tethers Unlimited in Lynnwood, Washington, is exploring the possibility of building a rotating, weblike network of tethers to hold a group of spacecraft in formation. Meanwhile, mechanical engineer Stephen Canfield of Tennessee Technological University in Cooksville and others are investigating using a whiplike tether to hurl a probe from Earth orbit toward another planet, eliminating the need for a booster rocket. "The idea is to store up a lot of energy by spinning the tether. Then you release that energy by briefly capturing a payload on the tether's end," Canfield says.